AC1600 connecting only 1 device

> I just have bought this rooter [...]

   New or used?  If used, did you do a settings reset before trying to
use it?

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual (at least).  Read.  Look for
"Reset".  Further reading might not hurt, either.

> [...] and I can't connect more than 1 device. [...]

   "can't" is not a useful problem description.  It does not say what
you did.  It does not say what happened when you did it.  As usual,
showing actual actions (commands) with their actual results (error
messages, LED indicators, ...) can be more helpful than vague
descriptions or interpretations.

   What is the "1 device"?  What is its IP address?

> [...] What went wrong? [...]

   If the C6250 was configured as a modem only ("bridge" mode), rather
than as a modem+router, then it could connect ony one device at a time
to your ISP.  A settings reset should restore it to its default
(modem+router) mode.